1. Field of the Invention
This invention relates to a bushing or nozzle for use in a continuous casting dip forming apparatus of the type in which a core wire is continuously introduced into a crucible through a bushing, so that molten metal in the crucible is accreted on the moving core wire to form a cast rod having a clad resulting from the solidified accreted metal.
2. Prior Art
A conventional dip forming apparatus comprises a housing defining a chamber through which a core wire is passed, and a crucible for holding a molten metal disposed above the housing. The crucible has an inlet at its bottom for introducing the core wire into the bath of molten metal. Generally, a bushing or nozzle is connected at opposite ends thereof to an outlet of the housing and the inlet of the crucible, respectively, to introduce the core wire into the crucible from the housing. The bushing is rubbed at its inner surface by the core wire moving therethrough and is heated to elevated temperatures by the heat from the molten metal in the crucible. Therefore, the bushing must have good wear-resistance and thermal resistance. For this reason, the conventional bushing has heretofore been made of various wear-resistant and heat-resistant materials. Japanese Patent Application Laid-open (Kokai) No. 50-70236 discloses one such bushing made of a molybdenum-based alloy. However, such a busing has the following disadvantages:
(1) since the bushing is heated by the molten metal in the crucible to high temperatures, for example, of 1100.degree. to 1200.degree. C. in the case where the molten metal is copper, the crystal of molybdenum becomes greater (for example, on the order of several hundred micron meters) and is subjected to intercrystalline crack due to the abrasion between the bushing and the moving core wire. Fine fragments are produced from the bushing as a result of the intercrystalline crack, and such fragments tend to adhere to the core wire moving through the bushing. Such fine fragments are present in an interface between the core wire and clad of the resultant cast rod. When such a cast rod is drawn into a wire of a reduced diameter, the drawn wire is liable to be cut or severed, so that the drawing operation can not be carried out efficiently. It is considered that when a fragment or foreign matter having a size of one third of the diameter of the drawn wire is present in the wire, the drawn wire is subjected to a cut.
(2) The operation of the dip forming apparatus is sometimes stopped, and the crucible is removed from the apparatus. In this case, the busing is allowed to cool to room temperatures. During the temperature drop of the bushing, the bushing is subjected to oxidation because molybdenum has a relatively poor resistance to oxidation. As a result, the bushing is varied in shape and therefore can not be used again.
(3) In use, the bushing is subjected to undue wear and seizing at its inner surface due to a friction between the bushing and the moving core wire. Therefore, the bushing must be replaced by a new one. The service life of the molybdenum-based bushing in the continuous operation of the apparatus is a relatively short time of 30 to 80 hours.